Phase transfer catalysis ("PTC") is a synthesis method which allows the use of relatively simple two-phase reaction systems in the place of solvent systems which may be toxic and/or expensive.
In fundamental terms, PTC employs a phase transfer catalyst which facilitates the transfer of a reactive species from the first phase, normally aqueous, into the second phase, normally organic, where the desired reaction can take place. For example, a phase transfer catalyst may be used to transport a cyanide ion from an aqueous solution of sodium cyanide to an organic solution of an alkyl halide where the cyanide ion reacts with the alkyl halide to form alkyl nitrile.
PTC is known to be a valuable technique for accomplishing a wide range of reactions. For example, PTC is known to facilitate nucleophilic substitution reactions, hydroxide transfer, esterification, oxidation and reduction. Specifically, U.S. Pat. No. 3,992,432 claims the use of certain quaternary salts as phase transfer catalysts to effectuate reactions such as displacement reactions (for example the earlier described preparation of alkyl nitrile), oxidation of organic compounds with inorganic oxidizing agents, ester saponification and the conversion of carbonyl compounds to alcohols. The quaternary salts for use as phase transfer catalysts taught and claimed in U.S. Pat. No. 3,992,432 are of the general formula (R.sub.1 R.sub.2 R.sub.3 R.sub.4 M).sup.+ X.sup.- wherein M is nitrogen, arsenic, phosphorus, antimony or bismuth; X is a halide or hydroxy ion; and R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are monovalent hydrocarbon radicals having a total sum of 18 to 70 carbon atoms, one of which may be further substituted by a quaternary group so the salt is represented by R.sub.1 [(R.sub.2 R.sub.3 R.sub.4 M).sup.+ X.sup.- ].sub.2. In Example 41 of U.S. Pat. No. 3,992,432, 2-octanone is reduced to 2-octanol by sodium borohydride in the presence of the phase transfer catalyst tricaprylylmethyl ammonium chloride.
Reduction of the aromatic compound C.sub.6 H.sub.5 (C:OX) (where X is halo and the benzene ring may be optionally substituted) to C.sub.6 H.sub.5 (CH.sub.2 OH) with an alkali or alkaline-earth metal borohydride in the presence of a phase transfer catalyst (which may be a quaternary ammonium compound) is known from GB 2 155 464 A.
Also, the article "N-Dodecyl-N-methylephedrinium Bromide: a Specific Catalyst for the Borohydride Reduction of Carbonyl Compounds under Phase-Transfer Conditions" (Stefano Colonna and Roberto Fornasier, Synthesis, August 1975, pages 531-2) compares the performance of five phase transfer catalysts in the reduction of 2-octanone to 2-octanol with sodium borohydride. Among the five phase transfer catalysts reported by Colonna et al. are N-dodecyl-N-methylephedrinium bromide and bis[2-hydroxyethyl]-dodecylmethylammonium bromide (referred to in Colonna et al. as "bis[2-hydroxyethyl]-dodecylmethylaminium bromide"). Colonna et al. also suggest on page 532 the presence of a hydroxy group in the catalyst enhances the rate of reduction. In a separate article, (Journal of the Chemical Society Perkin Trans I, "Asymmetric Induction in the Borohydride Reduction of Carbonyl Compounds by Means of Chiral Phase-transfer Catalysts. Part 2.", 1978, pages 371-3). Colonna and Fornasier report that to achieve asymmetric induction in the borohydride reduction of carbonyl compounds under phase-transfer conditions, the hydroxy-group must be in the .beta. position to the `onium` function and the catalyst must be conformationally rigid.
The above-mentioned references do not disclose that N-benzyl-N,N-bis(2-hydroxyethyl)cocoammonium halides are superior phase-transfer catalysts in processes for reducing carbonyl compounds by an alkali or an alkaline-earth metal.
Surprisingly, it has been found that in the alkali or alkaline-metal earth borohydride reduction of carbonyl compounds use of an N-benzyl-N,N-bis(2-hydroxyethyl)cocoammonium halide as a phase transfer catalyst provides a more rapid reduction reaction when compared with known phase transfer catalysts. The process of the current invention is particularly useful in the reduction of ketones to the corresponding alcohols, such as the reduction of 2-octanone to 2-octanol.